1. Field of the Invention
The present invention relates generally to an optical fiber and, more particularly, to an optical fiber having a Bragg grating, which allows light in a specific wavelength band to be selected out of light propagating through the core of the optical fiber by reflecting light having a wavelength that satisfies a Bragg condition and transmitting light having a wavelength that does not satisfy the Bragg condition.
2. Description of the Related Art
Generally, a fiber Bragg grating is manufactured utilizing an optical refractive index modulation effect that is generated when Ultra Violet (UV) rays are irradiated onto a core of the silica optical fiber in which impurities, such as germanium, boron, and phosphorous are doped in the core of the optical fiber.
When light is allowed to be incident on the optical fiber having a fiber Bragg grating, light with a wavelength that satisfies a Bragg condition is reflected, and light with a wavelength that does not satisfy the Bragg condition is transmitted. The wavelength that satisfies the Bragg condition is referred as a Bragg wavelength or a resonant wavelength. The fiber Bragg grating using such characteristics is generally used as a filter to select light in a specific wavelength band, and has been attracting a lot of interest from several years ago until now, as an optical device having various application fields.
FIG. 1 is a diagram showing a conventional method of manufacturing the fiber Bragg grating.
Referring to FIG. 1, in the conventional fiber Bragg grating manufacturing method, a periodic interference pattern 142 is directly formed in the core 131 the optical fiber using a phase mask 120 and UV light 110. This method employs a diffractive optical element (the phase mask 120) to spatially modulate the UV light beam. The phase mask 120 may be formed either holographically or by electron-beam lithography. When viewed from the side, the periodic surface relief structure 121 has square wave shapes. To manufacture the fiber Bragg grating, the optical fiber 130 is located near the one surface of the phase mask 120 having the periodic surface relief structure 121 to such an extent that the optical fiber 130 almost comes in contact with the one surface of the phase mask 120. The UV light 110 perpendicularly irradiated onto the phase mask 120 is diffracted due to the periodic surface relief structure 121 of the phase mask 120.
The phase mask 120 is made from flat slab of silica glass which is transparent to UV (ultra-violet) light. On one of the flat surfaces, a one dimensional periodic surface relief structure 121 is etched using photolithographic techniques. The shape of the periodic pattern approximates a square wave in profile. The optical fiber 130 is paced almost in contact with the corrugations of the phase mask 120 as shown in FIG. 1. Ultraviolet light 110 which is incident normal to the phase mask 120 passes through and is diffracted by the periodic corrugations 121 of the phase mask 120.
Normally, most of the diffracted light 111, 112, and 113 is contained in the +1, 0, −1 diffracted orders. However, the phase mask 120 is designed to suppress the diffraction into the zero-order 112 by properly controlling the depth of the corrugations in the phase mask. In practice, the amount of light 112 in the zero-order can be reduced to less than 5% with approximately 40% of the total light intensity divided equally in the ±1 orders 111 and 113. The two ±1 diffracted order beams 111 and 113 interfere to produce a periodic pattern that photoimprints a corresponding grating in the optical fiber 130. If the period of the phase mask grating 120 is Λmask, the period of the photoimprinted index grating 141 is Λmask/2.
However, when the fiber Bragg grating is manufactured using the conventional method, the period of the phase mask grating 120 is constant, so that the resonant wavelength of the fiber Bragg grating is fixed. For example, if the predetermined resonant wavelength of a grating is λ1, the conventional method is problematic in that a new phase mask 120 having a new period must be used, or the optical fiber 130 used to manufacture the fiber Bragg grating operated at the wavelength λ1 must be replaced with a new optical fiber having another refractive index so as to manufacture a fiber Bragg grating operated at the new wavelength other than the wavelength λ1.
Accordingly, the conventional method is disadvantageous in that, since the optical fiber 130 or the phase mask 120 must be replaced with a new optical fiber or a new phase mask, so that additional costs are required only to adjust the resonant wavelength.